Place the unpeeled onions in the microwave and cook on a high heat for 10 minutes, or until soft.Put a large non-stick casserole pan on a medium-low heat with 1 tablespoon of oil. Pick over the sage leaves and cook until crispy, then transfer to a kitchen towel-lined plate, leaving the pan on the heat.Tip the beef into the pan and cook for 10 to 15 minutes, or until browned, breaking it up with your spoon.Pulse the mushrooms in a food processor until fine (or finely chop by hand), then add to the pan and cook for 15 to 20 minutes, or until golden brown, stirring occasionally.Preheat the oven to 180°C/350°F/gas 4.Once cool enough to handle, squeeze the onions out of their skins into a food processor and pulse until fine (or finely chop), then stir into the pan.Tip in the black beans (juices and all), along with 2 tins’ worth of water and cook for 20 minutes, or until thickened, then season to perfection with sea salt and black pepper.For the cobbler topping, place the flour in the food processor with the wholegrain mustard and a pinch of salt and pepper. Break over the butter, coarsely grate in the Cheddar, pour in the milk and crumble in half of the crispy sage (putting the rest aside for garnish). Pulse until combined and the dough comes away from the sides of the bowl.Transfer the dough to a clean, lightly flour-dusted surface and pat into an 18cm round, then cut into 8 wedges and sit them on top of the beef mixture. Bake in the oven for 20 minutes, or until the dumplings are golden and risen.Meanwhile, boil the kettle. Cut the carrots and swede into even-sized pieces, then cook in a large pan of boiling salted water for 15 minutes, or until tender. When there’s 5 minutes to go, wilt the spring greens in a colander over the pan, removing any tough stalks. Drain the carrot and swede, then return to the pan and mash with 1 tablespoon of oil, then season to perfection.Divide the cobbler between plates, scatter over the reserved crispy sage and serve with the mash and spring greens.